A few options:

my $string = "P5";

my $num = substr $string, 1;

# or

my( $num ) = $string =~ m/^\w(\d+)/;

# or

my $num;
$num = $1 if $string =~ m/^\w(\d+)/;
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...just to name a few. :) If your input is assured to begin with a capital letter, I see no reason in particular to bother checking to see if the first character is a capital letter. For that matter, I don't really even need \w, since you already know it starts with a letter. The RE could be rewritten as m/^.(\d+)/