You don't have fixed width of line, look behind or look ahead will only work on fixed width

use strict;
use warnings;

while(my $line = <DATA>)
{
    chomp($line);
    if( $line =~ /(?<!fee\/src\/)(com\/)/ )
    {
        print $line . "\n";
    }
}
__DATA__
foo/src/com/
foo/src/com/
foo/src/com/
fee/src/com/
[download]

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