When a script doesn't work as expected, you need to debug it. To aid in that venture, be verbose. Don't get fancy and try to do everything in a single line (eg. my $log_file = (split /\./, $0)[0] . '.log';). Rather, do the minimum possible in each step, and display all intermediate results, such as:

#!/usr/local/bin/perl -w
# my_script_name.pl
use strict;

my $arg0 = $0;
print "Debug>  Before split -- arg0 = '$arg0'\n";

my @words = split /\./, $0;
my $count = @words;
print "Debug>  After split -- $count words:  [@words]\n";

my $base = $words[0];
print "Debug>  First word (word[0]) = '$base'\n";

my $log_file = $base . '.log';
print "Debug>  Full logfile name = '$log_file'\n";
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You should never find yourself saying "I know the contents of the variable should be X at this point". Rather, "I think the contents of the variable should be X, so I'll print them out to be sure..."

Works for me with

This is perl, v5.8.4 built for i86pc-solaris-64int
(with 32 registered patches, see perl -V for more detail)

Copyright 1987-2004, Larry Wall
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But note that you are not really taking the script name, you are also potentially taking the whole path to the script name. And you are not replacing the end of the script name by .log but you are replacing the complete part after the first dot in the script path and replace it by .log. I would do it this way:

(my $logname = $0) =~ /\.pl$/.log/;
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you are replacing the complete part after the first dot in the script path and replace it by .log

Good point. And here is one way of calling the script to reproduce the OP's result:

$ ./my_script_name.pl
.log
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Perhaps the OP has invoked the script differently on the 2 OS's.

yes, this is exactly what happens, even setting PATH with the current dir and invoking with:

$ my_script_name.pl
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The value of $0 is still the same:

./my_script_name.pl
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With this, work fine (note the 's/')

(my $log_file = $0 )=~ s/pl$/log/;
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use strict;
use warnings;

print "script = >>>$0<<<\n";
my $log_file = (split /\./, $0)[0] . '.log';
print $log_file, "\n";
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What happens when you run the script under the debugger? The -d option is there for a reason.

Try File::Basename::fileparse().