Instead of deleting the unwanted part, you could split each line into parts and just use the wanted part.
#! /usr/bin/perl
use strict;
use warnings;
my $in_file = 'input.txt';
my $out_file = 'output.txt';
my @result;
open my $handle_in, '<', $in_file or die "$in_file: $!";
while ( <$handle_in> ) {
# split (into 2 parts) at = (enclosed in optional whitespaces)
# use the last part and store it in @result
push @result, ( split( /\s*=\s*/, $_, 2 ) )[-1];
}
close $handle_in;
# remove line breaks from array elements
chomp @result;
open my $handle_out, '>', $out_file or die "$out_file: $!";
# print elements (ignore undefined and empty ones)
print $handle_out join( '/', grep { defined && length > 0 } @result )
or die "$out_file: $!";
close $handle_out or die "$out_file: $!";
__END__
Considering ww's message you might need to add more checks and logic to match more complicated input files.