In that case then glob would find the files

#!/usr/bin/perl
use strict;

my $dir = 'c:/temp/tmp'; # directory to search
my @files = glob("$dir/*.txt");

for my $filename (@files){
  my $lineno = 0;
  print "Scanning $filename ..\n";
  open IN,'<',$filename 
    or die "Could not open $filename : $!";
  my $iface;
  while (<IN>){
    ++$lineno;
    if (/^interface\s+(.*)/){
      $iface = $1;
    }
  
    if (/spanning-tree guard/){
      print "$filename,$lineno,$iface,$_";
      $iface = '';
    }  
  }
}
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Just two nitpicks: glob splits its argument on whitespace, so $dir can't contain any whitespace - except if you do use File::Glob ':bsd_glob';, which I'd strongly recommend in this case. Also, filenames beginning with a dot will be ignored (which is probably fine but should be mentioned).