I would write that more simply as: 

my @compounds = qw(
    C3H8O2
    C6H10O5
    C36H62O31
    C5H16N2
    C10H11N5O6P
    C9H11N3O7P
    C3H9N2O2
    );

print
    map "$_\n",
    sort { ( $a =~ /C(\d+)/ )[ 0 ] <=> ( $b =~ /C(\d+)/ )[ 0 ] }
    grep /P/,
    @compounds;

[download]

And you could also use a ST as well: 

my @compounds = qw(
    C3H8O2
    C6H10O5
    C36H62O31
    C5H16N2
    C10H11N5O6P
    C9H11N3O7P
    C3H9N2O2
    );

print
    map "$_->[0]\n",
    sort { $a->[ 1 ] <=> $b->[ 1 ] }
    map /P/ ? [ $_, /C(\d+)/ ] : (),
    @compounds;

[download]

In reply to Re: Condensing a grep into a sort by jwkrahn
in thread Condensing a grep into a sort by seaver

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