$ cat in.txt 
1  2  3  delta
1  2  3  apricot
1  2  3  charlie
1  2  3  bravo
1  2  3  echo
1  2  3  fox

$ cat mysort.pl 
#!/usr/bin/env perl
use warnings;
use strict;

my @rows;
while ( my $line = <> ) {
	my @fields = split ' ', $line;
	push @rows, \@fields;
}
@rows = sort { $a->[3] cmp $b->[3] } @rows;
for my $row (@rows) {
	print join('  ', @$row ), "\n";
}

$ perl mysort.pl in.txt 
1  2  3  apricot
1  2  3  bravo
1  2  3  charlie
1  2  3  delta
1  2  3  echo
1  2  3  fox