G'day maxamillionk,

Prefer a lexical filehandle in as small a scope as possible. This has these benefits:

• There's no interference from another package variable called SOURCE.
• The filehandle only exists for a limited time; i.e. it doesn't persist outside the limited scope.
• Perl will automatically close the filehandle when the limited scope is exited.

There's a variety of ways to achieve this. Here's two:

$ perl -e '
    my $file = "pm_11157157_text";
    system cat => $file;
    my $version;
    {
        open my $fh, "<", $file;
        $version = (<$fh> =~ /(\d)/)[0];
    }
    print "Version: $version\n";
'
# CentOS Linux release 7.6.1810 (Core)
Version: 7
[download]

$ perl -e '
    my $file = "pm_11157157_text";
    system cat => $file;
    my ($version) = do {
        open my $fh, "<", $file;
        <$fh> =~ /(\d)/;
    };
    print "Version: $version\n";
'
# CentOS Linux release 7.6.1810 (Core)
Version: 7
[download]

You don't need release in the regex but you may want more than just "7" captured. Again, two possibilities:

$ perl -e '
    my $file = "pm_11157157_text";
    system cat => $file;
    my ($version) = do {
        open my $fh, "<", $file;
        <$fh> =~ /([0-9.]+)/;
    };
    print "Version: $version\n";
'
# CentOS Linux release 7.6.1810 (Core)
Version: 7.6.1810
[download]

$ perl -e '
    my $file = "pm_11157157_text";
    system cat => $file;
    my ($version) = do {
        open my $fh, "<", $file;
        <$fh> =~ /([0-9.]+.+$)/;
    };
    print "Version: $version\n";
'
# CentOS Linux release 7.6.1810 (Core)
Version: 7.6.1810 (Core)
[download]