use strict;
use warnings;

# ...

sub parse_from_handle {
  my ( $handle, ) = @_;

  while ( my $line = <$handle> ) {
    # parse lines here until a end condition strikes
    # ...
    last if $end_condition;
  }

  return # ...
}

# MAIN

my $file = shift @ARGV || die "no file given to read from\n";

open my $fh, '<', $file or die "open($file,ro) failed: $!";

# feed handle to subroutine
parse_from_handle( $fh );


close $fh;