# Main Code
my ($i_val, $a_regex);
# ...set $a_regex to some useful value (or not)
# ...set $i_val
execute($i_val, $a_regex);

sub execute {
  my ($i, $regex) = @_;
  return unless $regex;
  my $compiled = qr/$regex/;
  my $mib = "$oid{$i}$tftpserver";
  for my $host(@routers) {
    chomp $host;
    next if $host !~ /$compiled/;
    ...
  }
}

##</code><code>##

sub execute {
  my ($i, $regex) = @_;
  return unless $regex;
  my $mib = "$oid{$i}$tftpserver";
  for my $host(@routers) {
    chomp $host;
    next if $host !~ /$regex/o;  # HERE
    ...
  }
}