prefix_print("A\nB");
prefix_print("C\n");
[download]

use strict;
use warnings;

my $prefix = 'P';

{
    my $first = 1;

    sub prefix_print {
        print $prefix if($first);
        print map {
            join($prefix, split(/^/,$_,-1));
        } @_;
        $first = $_[-1] =~ m/\n$/;
    }
}

prefix_print("A\nB","C\n");
prefix_print("A\nB");
prefix_print("C\n");
[download]

Which gives:

PA
PBC
PA
PBC
[download]

Update 2: then, after receiving the inevitable bug report, I might try:

use strict;
use warnings;

my $prefix = 'P';

{
    my $first = 1;

    sub prefix_print {
        do {
            print $prefix if($first);
            $first = ($_ =~ m/\n$/);
            print join($prefix, split(/^/,$_,-1));
        } foreach @_;
    }
}

prefix_print('');
prefix_print("y");
prefix_print("A\nB","C\n");
prefix_print("A\nB");
prefix_print("C\n");
prefix_print("X","Y","Z","\n","A","B","C");
prefix_print("\n");
prefix_print("\n");
prefix_print('');
prefix_print();
prefix_print("the end\n");
[download]

Which gives:

PyA
PBC
PA
PBC
PXYZ
PABC
P
Pthe end
[download]