use Digest::MD5 qw(md5_base64);

# assume @files contains paths to all files (masters and possible dups)

my %cksum;

for my $file ( @files ) {
    my $size = -s $file;
    local $/;
    open( I, $file );
    my $md5 = md5_base64( <I> );
    push @{$cksum{"$md5 $size"}}, $file;
}

for ( keys %cksum ) {
    print "dups: @{$cksum{$_}}\n" if ( @{$cksum{$_}} > 1 );
}