You put my $dmg inside the if block. The my part tells it that when that block ends at the next } to destroy your $dmg variable. In other words teh my part tells it that the variable $dmg should only be available for the current block. To fix it add my $dmg; outside the if and remove the my from inside the block.

my $dmg;
if($room==6){
    $dmg=int (rand 5)+1;
}elsif($room==3){
    $dmg=int (rand 4)+1;
}elsif($room==2){
    $dmg=int (rand 3)+1;
}elsif($room==1){
    $dmg=int (rand 2)+1;
}
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That might be better written as