You might not want to use $a and $b, because those are built-in globals used by certain functions (eg sort).

Assuming you're looping over each line:

my @pairs;
while (<$filehandle>) { # or whatever loop construct
  # parse line and set $foo and $bar here

  # for example:
  chomp;
  my ($foo, $bar) = /(\d+)/g;

  # add to array
  push @pairs, [ $foo, $bar ];
}

# sort array
# NB: these are the special $a and $b, not yours
@pairs = sort { $$a[0] <=> $$b[0] } @pairs;
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