Rather than seeking to the end and then reading backwards a byte at a time, it might be quicker to read a reasonable (greater than line length) sized chunk from the end of the file and then let the regex engine locate the last line.

#! perl -slw
use strict;
our $MAX ||= 512;

open TAIL, '< :raw', $ARGV[ 0 ] 
    or die "$ARGV[ 0 ] : $!";
sysseek( TAIL, -$MAX, 2 );
sysread( TAIL, my $buffer, $MAX );
my( $line ) = $buffer =~ m[\n(.*?$)];

print $line;
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