You don't need to run through an array (or hash) here. You're only matching once per script run, and an index call will work much more efficiently:

use strict;
use warnings;

my $url = "http://somes.erver/some/file.avi";
my ($ext) = $url =~ /\.(\w+)$/;

my $quicktime_ext = 
'.sdp .rtsp .rts .mov .qt .smi .sml .smil .avi .vfw ' .
'.flc.fli .wav .bwf .aiff .aif .aifc .cdda .au .snd ' .
'.ulw .mid .midi .smf .kar .qcp .sd2 .amr .gsm .mpeg ' .
'.meg .m2s .m1v .ma1 .m75 .m15 .mpm .mpv .mpa .3gp  ' .
'.3gpp .3g2 .3gp2 .mp4 .mpg4 .m4a .m4p .m4b .m4v  ' .
'.sdv .amc .swa .m3u .m3url .swf .fpx .fpix .dv .dif ';

 print 'Valid ext' if $ext && index($quicktime_ext, ".$ext ") != -1;
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