Without looking too much into it, a fast Q&D would be as follows:

# Test line
$line = "------------------- Frame 1 ------";

# Parse it by changing all -, letters, and spaces into nothing
$line =~ s/[\-|A-Z|a-z|\s+]//g;

# Print the line which returns '1' or whatever number it might be
print "$line\n";
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Hope this helps! There's more extravagant ways to do it but that works effectively to solve your situation.