for (my $c = 1;; $c ++) {
    my $file = "graph_set_$c.out";
    last unless -f $file;
    open my $fh => "<", $file or die "Failed to open $file: $!";
    ...  # Do something with $fh.
}
[download]

Abigail

I have added the code that you have supplied.
what I want to do is match the specified strings from each graph_set.out file and print strings into .txt file.
It's not printing anything into the .txt file ? ?

#!/usr/local/bin/perl 

for (my $c = 1;; $c ++) {
    my $file = "graph_set_$c.out";
    last unless -f $file;
    open my $fh => "<", $file or die "Failed to open $file: $!";

local $/ = undef;       # undef record seperator
my $string = <IN>;         # read whole file into string    
close (IN);

my @gset_match = ($string =~ /([A-Z]\s\d+,\s\d+\(\s*\d+\))/g);

open (TEXT, ">graph_set.txt") or die "Can't create graph_set.txt: $!\n
+";
foreach $_(@gset_match) { print TEXT "$_\n"; }
close TEXT; 
}
[download]

You are reading from a handle named IN, while the open opens a handle $fh. That isn't going to work. If you turn warnings on (which usually is a good idea), Perl would have told you that you were reading from an unopened filehandle.

Abigail

use strict;
use warnings;

use IO::File;

foreach my $i (1 .. 30)
{
    my $fh = IO::File->new("graph_set_$i.out")
        || die "Cannot open $i'th file\n";

    # Use $fh here

    $fh->close;
}
[download]

------
We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.

There are two steps to this. First you need to get a list of file names and sort them in the desired order. Then, open the files in a for loop over the sorted list.

my $path = '/path/to/dir';
my @names = glob $path . '/graph_set_*.out';

# @names is in alpha order, so sort numerically
my @sorted_names =
    sort {
        my ($x, $y);
        $a =~ /graph_set_(\d+)\.out$/ and $x = $1;
        $b =~ /graph_set_(\d+)\.out$/ and $y = $1;
        $x <=> $y
    } @names;
# That may benefit from a Schwartzian Transform

# Now, do the loop
for (@sorted_names) {
    open local(*IN), "< $_" or die $!;
    # ...
    close IN or die $!;
}
[download]

After Compline,
Zaxo

Rather than trying to open 30 files when some or all of them may not exist, and dieing if one of them doesn't (unless that makes sense). You could let perl tell you which files exist and just open those that do.

while( my $file = <graph_set_*.out> ) {
    next unless $file =~ m[graph_set_[123]?[0-9].out];
    open IN, '<', $file 
        or warn "Couldn't open file $file because $!";
    #Do stuff
    close IN;
}
[download]

The caveat is that you may not get the files in the order you want them if that is important. They will be in sorted order (as of 5.8 I think) but as your numbers don't appear to have leading zeros, they will come in ascii order 1 10 11 12 13 14 15 16 17 18 19 2 20 21 22 23 24 25 26 27 28 29 3 30 4 5 6 7 8 9

If that's a problem, then you could do

for $file (sort{ $a=~/(\d+)/ <=> $b=~/(\d+)/ } <graph_set_*.out> ) {
    open...
    ....
    close...
}
[download]

there is nothing printing into the txt file.
is there anything obviously wrong ?
I know the pattern matching code is right.

#!/usr/local/bin/perl 

while( my $file = <graph_set_*.out> ) {
    next unless $file =~ m[graph_set_[123]?[0-9].out];
    open IN, '<', $file 
        or warn "Couldn't open file $file because $!";
  
local $/ = undef;       # undef record seperator
my $string = <IN>;         # read whole file into string    
close IN;
}

my @gset_match = ($string =~ /([A-Z]\s\d+,\s\d+\(\s*\d+\))/g);

open (TEXT, ">graph_set.txt") or die "Can't create graph_set.txt: $!\n
+";
foreach $_(@gset_match) { print TEXT "$_\n"; }
close TEXT;
[download]

Had you turned on strict, Perl would have told you that where you match $string that variable is out of scope. What you do is, in a loop, read in each file, store the context in a lexical variable (which is gone at the next iteration of the loop). Then, outside the loop, after discarding the content of all the files, you do the matching.

But there's nothing to match against.

Abigail

#!/usr/local/bin/perl

# NOT TESTED

@files = map  {$_->[1]}
          sort {$a->[0] <=> $b->[0] }
          map  { /^graph_set_([[0-9]+).out$/; [ $1, $_ ] }
          grep { /^graph_set_[0-9]+.out$/ }
          glob('graph_set_*');
# lookup schwartzian transform, then read the preceding from the botto
+m up

for (@files)
{
  open(F, $mode, $_);
  # do whatever
  close F;
}
[download]