I’m not sure about whether you are asking any question at all, but it seems you need to allow some whitespace between the square brackets in the data and the digits:

my @numbers = $file_content =~ /\[\s*(\d+)\s*\]/g;
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FWIW, I suggest using the /x option for the pattern, so that you can use whitespace to structure it:

my @numbers = $file_content =~ / \[ \s* (\d+) \s* \] /gx;
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See perlretut in Building a regexp.

Makeshifts last the longest.