use strict;

open( IN, "ICD_MEAS_child.txt" ) or die "failed to open child data: $!\n";
my @children = <IN>;  # read them all at once;
close IN;

open( IN, "ICD_MEAS.txt" ) or die "failed to open parent data: $!\n";
while (<IN>) {
    if ( /,C,/ ) {
        my $child = shift @children;
        s/\n/,$child/;  # update: added the essential comma
        s/,/;/g;  # update: added this line, as per orig. post
    }
    print;
}
close IN;