You can use typeglobs:

use strict;

print myslurp();

sub myslurp {
 if(shift){
  open(F,"<$_") or die "Can't open $_: $!";
 }else{
  F=*STDIN{IO};
 }
 local $/=undef;
 my $slurp=<F>;
 close(F);
 $slurp;
}
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