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Use capturing. Also take advantage of the fact the m// operator returns captures ($1, $2 etc.) in list context, and observe you don't need square brackets around \d:

my $text = "Signal\nLTE-FDD 4; Channel 2175";

my ($band, $channel) = ($text =~ /Signal\nLTE-FDD (\d{1,2}); Channel (
+\d{3,4})/i);

print "$band\t$channel\n";
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More on capturing groups can be found in perlre#Capture groups.