use IO::File ();
use IO::Scalar ();

sub do_stuff_with_a_fh {
  my ($fh) = @_;
  ...do stuff...
}

my $str = "this is my string";
my $fh_scalar = IO::Scalar->new(\$str)
  or die "failed to create fh from str: $!";

do_stuff_with_a_fh($fh_scalar);

my $fh_stdin_duped = IO::File->new("<&STDIN")
  or die "failed to dup stdin: $!";

do_stuff_with_a_fh($fh_sdtin_duped);