#! perl

use warnings;
use strict;

use File::Temp qw( tempfile );

# this function encapsulates the technique.
# it returns a function that can be used to
# inject input into the targeted file handle

sub inject_input {

    my $target_fh = shift;
    my ($temp_fh, $temp_fn) = tempfile();
    my $temp_fd = fileno $temp_fh;

    local *SAVED;
    local *TARGET = $target_fh;
    local *INJECT;
    open SAVED,  "<&TARGET"     or die "can't remember target";
    open TARGET, "<&=$temp_fd"  or die "can't redirect target";
    open INJECT, "+>>$temp_fn"  or die "can't open injector";
    unlink $temp_fn;  # we don't need the directory entry any more

    select((select(INJECT), $| = 1)[0]);

    my $saved_fh  = *SAVED;
    my $inject_fh = *INJECT;

    return sub {
        if (@_) {
            print $inject_fh @_;
        }
        else {
            seek $inject_fh, 0, 0 or die "can't seek";  # rewind
            my $injected_output = do { local $/; <$inject_fh> };
            close $temp_fh or die "can't close temp file handle";
            local (*SAVED, *TARGET, *INJECT)
                = ($saved_fh, $target_fh, $inject_fh);
            open TARGET, "<&SAVED"  or die "can't restore target";
            close SAVED             or die "can't close SAVED";
            close INJECT            or die "can't close INJECT";
            return $injected_output;
        }
    }
}