I would restructure the code to be less clever but easier to debug:

if( $html !~ m!<span class="remaining-data">\s*(.*?)used\s*</span>! ) 
+{
    die "Couldn't find 'remaining-data' span in html!";
};
my $remaining = $1;
if( $remaining !~ m!^(\d+(?:\.\d+))(MB|GB)$! ) {
    die "Weirdo string instead of remaining data: [$remaining]";
};
my ($DataUsed, $unit) = ($1,$2);
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