The greedy death-star is another problem. You need to limit it:

C:\>perl -E "my $num ='abcd23.5fghi';if ($num =~ /[a-z]+(.*?)[a-z]/i){
+ say $1;}"
23.5                                                      /^\
[download]

The above is OK (Yes, I know OP wants only INTs, but this is a minimal example of the death-star problem) when the greedy ".*" is limited by the "?" but it's definitely not what the OP seems to want if the limit ("?") is omitted.

C:\>perl -E "my $num ='abcd23.5fghi';if ($num =~ /[a-z]+(.*)[a-z]/i){ 
+say $1;}"
23.5fgh    # not what OP seems to want
[download]

Also rather than the clumsy "/\d{1}-\d{2}/" you might wish to use "/[0-9]+/" and, more verbosely this also works (but is a far less stringent regex):

C:\>perl -E "my $num ='abcd23fghi'; my $intermediate; if ($num =~ /[a-
+d]+(\d+)[f-z]+/) {$intermediate = $1;} if ($intermediate =~ /[0-9]+/ 
+){ say $intermediate;}"
23
[download]

... whereas this fails:

C:\>perl -E "my $num ='abcd23.777fghi'; my $intermediate; if ($num =~ 
+/[a-d]+(\d+)[f-z]+/) {$intermediate = $1;} if ($intermediate =~ /[0-9
+]+/ ){ say $intermediate;}"
(no output)
[download]

The previous discussion should make it easy to identify the cause of the failure.