First of all please try to follow basic guidelines in your code.

• Check if open() actually worked.
• Use consistent indentation to make the codeflow easier to follow (perltidy can do this for you).
• You should use strict; use warning;
• For better scoping, put open and close within the sub
• Subs are generally called like findLines();
• Use named variables instead of the implicit $_

Here's some pseudo-code on how to return an array:

use strict;
use warnings;

use Data::Dumper;

sub findLines {
    open(my $error_fh, '<', 'iset_error_log') or die($!);
    my @lines;
    while((my $line = <$error_fh>)) {
  
        next unless($line =~ /\[notice/); skipp all lines except notic
+e

        # yada, do something to $line

        push @lines, $line;
    }

    close($error_fh);
    return @lines;
}

my @found = findLines();
print Dumper(\@found);
[download]

To be more memory efficient, you could also return a reference to the array, like this:

    # .....
    close($error_fh);
    return \@lines;
}

my $found = findLines();
print Dumper($found);
[download]