Nooooo! :) See Death to dot star to read why .* is bad.
You can't use \1 there, you should just use 1.
Please note that /^1/ will also match strings beginning with 123.

I suggest the following solution:

$line =~ s/$/,P/ if /^1\D/;
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Which will substitute the end of the line (which is _before_ a newline if there is any) with ,P (and thus "append" it) if the line starts with 1 followed by anything-that-is-not-a-digit.

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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