use strict;
use warnings;

use IO::Select;

...

my $text;
if ( @ARGV )
{
    if ( -r $ARGV[ 0 ] )
    {
        $text = do {
           local $/;
           <>;
           };
    }
    else
    {
        $text = join q{ }, @ARGV;
    }
}
else
{
    my $select = IO::Select->new();
    $select->add( \ *STDIN );
    print
       qq{Type text, finish with ctrl-D on its own line\n}
       unless $select->can_read( 0.05 );

    $text = do {
       local $/;
       <>;
       };
}

...

# Do something with $text