Also see a similar program Online GREP problem.

I would also suggest to pass the path to the file as an argument. You can always define a default as a constant in the top of your script where it is easy to find and change if needed. That's more maintainable than hardcoding a path in a subroutine

use constant DEFAULT_FILE => "/etc/filesystems";
my $file = shift || DEFAULT_FILE; # e.g. to take file from command lin
+e or use the default
find_pattern($file, $pattern);

sub find_pattern {
  my ($file, $pattern) = @_;
  # your code here
}
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Another general advise would be to separate the test and the action. Ideally, a subroutine "find_pattern" should return true if it found it and false if it didn't. For example next time you might want to terminate a script rather than print to STDOUT:

...
die "oh no, it's not there!" unless find_pattern($file, $pattern);

sub find_pattern {
  my ($file, $pattern) = @_;
  return `grep $pattern $file`;
}
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Also, as a general rule: if a subroutine opens a file, it should be responsible for closing it. A subroutine shouldn't leave a file open when it returns. Explicit closes are not required in the main program as all open file handles are closed when the main program exits. But if you pass a file name to something like my find_matching_lines() subroutine, you should close that file when you are finished with it, otherwise open the file the main program and pass a filehandle to the subroutine.

#!/usr/bin/perl -w
use strict;

print "*the backtick cat results:*\n";
print `cat /etc/filesystems`;

#prints:
#  ext3
#  ext2
#  nodev proc
#  nodev devpts
#  iso9660
#  vfat
#  hfs
#  hfsplus
print "\n"; #spacer line

my @catlines = `cat /etc/filesystems`;
print "*the catlines array:*\n";
print @catlines; #prints same as backtick results above
print "\n"; #spacer line

my $file    = '/etc/filesystems';
my $pattern = 'nodev';

my @lines= `grep $pattern $file`;
print "*the grep backtick results:*\n";
print @lines;
print "\n"; #spacer line

#prints:
#  nodev proc
#  nodev devpts

my @getlines = find_matching_lines($file,'ext');
print "*the getlines array*\n";
print scalar(@getlines), " lines found\n";
my $numlines = @getlines;
print "number of lines = $numlines\n"; #same thing
print "".@lines," lines found\n"; #the concatentation op "." forces sc
+alar
print @getlines;

sub find_matching_lines
{
   my ($file_name, $pattern)=@_;
   my @matches =();
   open (my $in, '<', $file_name) or die "unable to open $file_name";
   while (<$in>)
   {
      push (@matches,$_) if m/$pattern/;
   }
   close ($in);
   return @matches;
}

__END__
*the backtick cat results:*
ext3
ext2
nodev proc
nodev devpts
iso9660
vfat
hfs
hfsplus

*the catlines array:*
ext3
ext2
nodev proc
nodev devpts
iso9660
vfat
hfs
hfsplus

*the grep backtick results:*
nodev proc
nodev devpts

*the getlines array*
2 lines found
number of lines = 2
2 lines found
ext3
ext2
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