Then you can do something like:

my $file = $ARGV[0];

if (defined $file) {
   open(IN, "< $file")
   or die("Unable to open supplied argument: $!\n");
} else {
   *IN = *STDIN;
}

while (<IN>) {
   print($_); # do stuff
}
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or with some magic:

my $file = @ARGV ? $ARGV[0] : '-';

open(IN, "< $file")
or die("Unable to open input file: $!\n");

while (<IN>) {
   print($_); # do stuff
}
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or the OO way:

use FileHandle;

my $file = $ARGV[0];

my $fh_in;
if (defined($file)) {
   $fh_in = FileHandle->new($file, 'r')
   or die("Unable to open supplied argument: $!\n");
} else {
   $fh_in = *STDIN{IO};
}

while (<$fh_in>) {
   print($_); # do stuff
}
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