Here are two examples that does the same thing. I think the second one is easier to understand for someone new to Perl.

while(<>){
    print "$1;\n" if /^(Key [0-5] 0x[\da-f]{14})/i;
}

while( my $line = <>){
    if( my ($match) = $line =~ /^(Key [0-5] 0x[\da-f]{14})/i ){
        print "$match;\n";
    }
}
[download]

or maybe you want a one-liner:

perl -ni -le 'print "$1;" if /^(Key [0-5] 0x[\da-f]{14})/i' file.txt
[download]

This will change "file.txt".