I think this will do what you want.

#!/usr/bin/perl
# 2005-03-01:
# Please always use strict and warnings, they will capture
# many common mistakes and typos
use strict;
use warnings;

# we may as well test it before we try to open it
unless (-T 'imagescartes.txt') {
     print "ceci n'est pas un fichier texte\n";
     exit 1;
}

# ouvrir le fichier contenant la liste des cartes
open CARTES, '<', 'imagescartes.txt' or die 'Ouverture du ficher image
+scartes.txt impossible: $!\n';

# this is the array where we store the image file names
my @amourss;
while (my $ligne = <CARTES>)
{
    chomp $ligne ;
    if ($ligne =~ /^a\S+\.(?:gif|jpg)$/) # will match exactly one word
                                         # begining a and ending in .g
+if or .jpg
                                         # the \S+ is one or more non 
+space chrs
                                         # a.gif not allowed use \S* t
+o allow it
    {
        push @amourss, $ligne;
    }
}

close CARTES;

print "found ", scalar @amourss, " images: ";
print join ", ", @amourss;
print "\n";
__END__

# input file used 
test.gif
this asilly.gif
abc.jpg
abcd.gif
absolutely not a.jpg

# results of running 
>./amourss
found 2 images: abc.jpg, abcd.gif
>
[download]

Update

as Jasper points out a space is valid in filenames in most OSes so please feel free to change the regex to /^a.+\.(?:gif|jpg)$/ if you wish to allow spaces in filenames or /^a.*\.(?:gif|jpg)$/ if you wish to allow a.gif and a.jpg

Cheers,
R.