This is a somewhat confusing question. the first argument to a perl script is in $ARGV[0]. so you can do this.

use strict; use warnings; 
die 'I need an argument' unless ($ARGV[0]); 

#
# capture name of file 
# 
my $name_of_file=$ARGV[0]; 

#
# call a subroutine with the name of file 
#   And capture the data it returns 
# 

my $data=do_something_else ($name_of_file); 
exit; 

sub do_something_else {
  my $filename=shift;  
  print 'Name of file:'.$filename."\n"; 
#
# read all of file at once into single variable 
# 

  my $file_contents; 
  { 
    local $/=undef;
    open (my $infile,'<',$filename) or die 'cannot open'.$@; 
    $file_contents=<$infile>; 
    close ($infile); 
  } 
  print 'size of file:'.length($file_contents)."\n"; 
  return $file_contents; 
  } #do_something else
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Result:

perl argin.pl argin.pl
Name of file:argin.pl
size of file:684
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