You're comparing the encoded form of the character with the character. Fix:

#!/usr/bin/perl -w
use strict;
use utf8;

open(my $fh, '<:encoding(UTF-8)', $ARGV[0])
    or die "Can't read $ARGV[0]: $!\n";

my $fileln = <$fh>;

$fileln =~ tr/’/'/;

...[ do something with $fileln ]...
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In case you don't realize it, you didn't save your changes anywhere or do anything with them.